Routine service on many types of machines and vehicles involves checking the status of the brakes, in particular, the wear of brake pads or disks forming part of the brakes. Much effort has been made by designers to arrive at a good method for measuring brake wear. Additionally, there are regulations in some areas that mandate regular capacity checks on park brakes.
The issue, especially in wet brakes used in construction equipment having final drives embodying speed reduction gearing, is that checking the amount of wear visually or mechanically is difficult due to the brakes being housed at a location inboard of the final drive gears, wheel drive hub and spindle or axle. Tests to determine the reliability of parking brakes are becoming routine but have the drawback that most are pass/fail type tests with a failure requiring that the machine be shut down until the condition giving rise to the failure is remedied.
It is known to monitor the wear of a disc brake pack forming part of a park and service brake assembly used to brake a wheel of an industrial, off-road vehicle without requiring the disassembly of the brake assembly. This monitoring is done by using a depth gauge to measure the movement of the brake piston required for engaging the disc brake pack when the latter is new and comparing this value with subsequent measurements made during the service life of the disc brake pack. If the difference between the two measurements is within a specified wear limit, the disc brake pack need not be replaced, but if the difference exceeds the wear limit, new disc plates are required. Such a brake monitoring arrangement is disclosed in U.S. Pat. No. 4,186,822, issued Feb. 5, 1980. This wear measurement arrangement has the disadvantage that the brake piston for effecting engagement of the brake disc pack must be located so as to be accessible for permitting its movement to be manually measured, thus placing design constraints on where a park and service brake assembly may be placed when used with a final drive arrangement. This wear measurement has the further disadvantage of requiring the operator to dismount the cab and manually perform measurements, which is time-consuming.
Another known way of monitoring the wear of a vehicle wheel disc brake pack is to use an electronic control unit which receives wear value output signals from a distance sensor mounted on one or more brake lining supports, which measures the distance of the mount from the braked element. The electronic control unit has in memory an allowable wear value to which the measured wear value is compared, with the control unit emitting a warning signal when the measured wear value equals the allowable wear value. Also, the measured wear value can be indicated in a wear indicator apparatus. A less direct way of measuring wear is by storing a family of characteristics in the memory of the electronic control unit which correlate the brake lining temperatures, brake lining thickness and strength of the electronic signal fed to a brake torque control apparatus. On the basis of this stored family of characteristics, the electronic control unit forms a wear value signal from the strength of the signal fed to the braking torque control apparatus and the indicated brake lining temperature, which wear value signal characterizes the thickness and thus the wear of the brake lining. Thus, in a sense, the temperature sensors are also wear value transmitters, the transmitted wear value signal being converted into readings on a scale, if desired. U.S. Pat. No. 4,790,606 discloses such a wear monitoring apparatus.
It is also known to determine the integrity of a vehicle wheel braking device in an arrangement wherein the torque producing capability of the drive system is sufficiently large to override the braking toque produced. In this arrangement, the brake is first applied, and then sufficient torque is applied to the drive shaft to cause the brake to slip so that the drive shaft rotates a predetermined rotational distance, one revolution for example, about its axis. Slipping the brake causes relative motion between the brake plate and the reaction plate. This relative motion generates a braking torque between a brake-applying member and the reaction plate, the relative motion being measured and compared to reference values to verify brake functional integrity. U.S. Pat. No. 5,785,158 discloses such a brake integrity monitor. This manner of checking brake integrity has the drawback of requiring a sensor arrangement for determining the relative rotation between the brake-applying member and the reaction plate, which adds additional cost to the final drive arrangement.
What is desired then is some way to be able to reliably and economically measure wear of a disc brake pack of a vehicle service and park brake arrangement embodied in an electric motor driven final drive in a location making it difficult to visually or mechanically inspect the disc brake pack and to predict brake failure so that a customer is alerted to the need for servicing the brakes in order to avoid brake failure.